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By using the String class, we can make unmodifiable strings, i.e. immutable strings, but by using the StringBuffer class we can not only create strings of characters but can also modify them, which makes them mutable strings. The StringBuffer class is also a thread-safe class because its methods are marked synchronized.

When StringBuffer class is used?

• StringBuffer class is used in the creation of thread-safe applications, as its methods are marked with synchronized keyword, which makes them access by only by one thread at a time.


• Strings created by StringBuffer class are modifiable, hence this class is used to create an application that requires creation and modification of a large volume of strings.

Constructors of StringBuffer class

• public StringBuffer()

  Creates a StringBuffer object with initial capacity of 16 characters.
  



• public StringBuffer(String str)

  Creates a StringBuffer object using a String str.
  



• public StringBuffer(int capacity)

  Creates a StringBuffer object with a defined capacity of characters.

Creating an empty StringBuffer object

• In the upcoming example, we will create an empty StringBuffer class object by calling its constructor that takes no value.
• Next, we will modify this empty StringBuffer object by calling its append() method, to add some String values to it.

// Java - StringBuffer Class Example


class StringBufferEx
{
public static void main(String... ar)
{
	//Creating an empty StringBuffer object.
	StringBuffer sb= new StringBuffer(); 
	
	//Printing the value of StringBuffer object
	System.out.println("Original StringBuffer value : "+ sb);
	
	//Appending a String to the StringBuffer object
	sb.append("Stay");
	
	//Appending another String to the StringBuffer object
	sb.append("Positive");
	
	//Printing the modified value of StringBuffer object
	System.out.println("Modified StringBuffer value : "+ sb);
}
}

Output is :

Original StringBuffer value is : 
Modified StringBuffer value is : StayPositive

Program Analysis

• In the example, we have first created an empty StringBuffer object and have printed its empty value.
• Next, the append() method of StringBuffer class is called to append a String, Stay to the StringBuffer object.
• Next, the append() method is called again to append a String, Positive to the StringBuffer object.
• Finally, we have printed the modified value of StringBuffer object, KeepSmiling.

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Creating and initializing a StringBuffer object using a String

• In the next example, we will first create and initialize a StringBuffer class object by calling its constructor that takes a String value.
• Next, we will modify this StringBuffer object by calling its append() method, to add new String values to it.

// Java - StringBuffer Class Example

class StringBufferEx
{
public static void main(String... ar)
{
	//Creating and initializing a StringBuffer object using a String value
	StringBuffer sb= new StringBuffer("Keep");

	//Printing the value of StringBuffer object
	System.out.println("Original StringBuffer value : "+ sb);
	
	//Appending a String to the StringBuffer object
	sb.append("Smiling");
	
	//Printing the modified value of the StringBuffer object
	System.out.println("Modified StringBuffer value : "+ sb);
}
}

Output is :

Original StringBuffer value : Keep
Modified StringBuffer value : KeepSmiling

Program Analysis

• In the example, we have first created a StringBuffer object which is initialized with a String value, Keep.
• Next, the append() method of StringBuffer class is called to append a String Smiling to the StringBuffer object.
• Finally, we have printed the modified value of our StringBuffer object i.e. KeepSmiling.

Finding the index of a substring in a string created by StringBuffer

In the next example, we will be finding the index of a substring within in a string created by the StringBuffer class by calling its indexOf method.

// Java - Example of StringBuffer class

class A
{
public static void main(String... ar()
{
	//Creating a StringBuffer object
	StringBuffer sb= new StringBuffer("Blue"); 
	
	//Printing the StringBuffer object
	System.out.println("Original StringBuffer value : "+ sb); 
	
	//Finding the index of substring "ue" in StringBuffer string
	int index = sb.indexOf("ue"); 
	
	//Printing the index position of a substring "ue" in StringBuffer string 
	System.out.println("index of ue  in StringBuffer string  : "+  index);
}

Output is :

Original StringBuffer string value : Blue
index of ue in StringBuffer string : 2

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